Magnetic resonance imaging scanner with booster iron

ABSTRACT

A magnetic resonance imaging scanner includes a magnet ( 20 ) generating a temporally constant magnetic field, magnetic field gradient-generating structures ( 30 ) superimposing selected magnetic field gradients on the temporally constant magnetic field, and a radio frequency coil ( 32 ) producing a radio frequency field. A magnetic field-modifying structure ( 60 ) disposed inside a radio frequency shield ( 64 ) includes dispersed particles of magnetic material ( 701, 702, 703, 704 ) that enhance the temporally constant magnetic field. The particles are generally smaller in at least one dimension than a skin depth of the radio frequency field in the magnetic material. The magnetic field-modifying structure has a longitudinal demagnetization factor (Nz) parallel to the temporally constant magnetic field and a tangential demagnetization factor (NT) in a tangential direction transverse to the temporally constant magnetic field. The longitudinal demagnetization factor is larger than the tangential demagnetization factor to produce tangential flux guiding.

The following relates to the magnetic resonance arts. It findsparticular application in magnetic resonance imaging, and will bedescribed with particular reference thereto. However, it also findsapplication in magnetic resonance spectroscopy and other techniques thatbenefit from a substantially uniform main B₀ magnetic field.

Magnetic resonance imaging scanners with magnet bores that are short inthe axial or z-direction reduce patient claustrophobia and can provideimproved access to the patient for interventional procedures. A shortbore magnet may, for example, have bore length of less than 1.5 meters,or less than 1 meter. Short-bore magnets, however, typically havedegraded static main B₀ magnetic field spatial uniformity as comparedwith longer bore magnets, due to field bending at the ends of the shortbore.

One approach for improving field uniformity is the use of “booster”iron. In this approach, a magnetic field-modifying structure includesiron or another ferromagnetic material that is coupled with the main B₀magnetic field. The magnet coils are designed in conjunction with themagnetic field-modifying structure, such that the main magnet and themagnetic field-modifying structure together produce a substantiallyspatially uniform static main B₀ main magnetic field. The booster ironstretches the magnetic field to compensate for the reduced bore length.Moreover, as the booster iron is typically in saturation at typical mainB₀ magnetic field magnitudes (e.g., at 1.5 T or higher), the effect ofthe booster iron on the static main B₀ magnetic field is substantiallyindependent of magnetic field gradients. Instead of designing thebooster iron concurrently with the magnet, the booster iron can bedesigned empirically after magnet manufacture by adding iron to improveuniformity, stretch the field, or otherwise improve the main B₀ magneticfield in the already-manufactured magnet.

In some designs, the magnetic field-modifying structure is placedoutside of the radio frequency coil and the radio frequency shield thatshields surrounding structures from the radio frequency signals. Thisapproach substantially reduces interaction between the booster iron andthe radio frequency fields. However, placing the booster iron outsidethe radio frequency shield has certain disadvantages. Space constraintsin the scanner can make placement of the booster iron outside the radiofrequency shield difficult. Moreover, the booster iron is less effectiveat modifying the main B₀ magnetic field in the imaging region as thebooster iron is moved further away from the imaging region. Thus, morebooster iron is required, which occupies additional space in the bore.

Moving the booster iron closer to the imaging volume by placing itinside of the radio frequency shield is problematic. Inside the shield,the booster iron interacts with and can degrade the radio frequency B₁magnetic field. The skin depth in iron of the radio frequency B₁magnetic field at typical magnetic resonance imaging frequencies issmall, being typically of order 10-20 microns or less. Consequently, theradio frequency B₁ magnetic field is substantially expelled from theinterior of the booster iron. This field expulsion causes the radiofrequency coil to operate less efficiently, and can causenon-uniformities in radio frequency fields. Moreover, eddy currentsinduced in the booster steel by the radio frequency fields can producemagnetic field distortions, image artifacts, and detrimental heatinginside the scanner.

The present invention contemplates an improved apparatus and method thatovercomes the aforementioned limitations and others.

According to one aspect, a magnetic resonance imaging scanner isdisclosed. A magnet generates a temporally constant magnetic field. Oneor more magnetic field gradient-generating structures superimposeselected magnetic field gradients on the temporally constant magneticfield. A radio frequency coil is disposed inside of a radio frequencyshield and selectively produces a radio frequency field. A magneticfield-modifying structure is designed to enhance the temporally constantmagnetic field. The magnetic field-modifying structure is disposedinside of the radio frequency shield, and includes particles of magneticmaterial generally smaller in at least one dimension than a skin depthof the radio frequency field in the magnetic material dispersed in aninsulating binder.

According to another aspect, a magnetic resonance imaging scanner isdisclosed. A magnet generates a temporally constant magnetic field. Oneor more magnetic field gradient-generating structures superimposeselected magnetic field gradients on the temporally constant magneticfield. A radio frequency coil selectively produces a radio frequencyfield. A magnetic field-modifying structure is designed to enhance thetemporally constant magnetic field. The magnetic field-modifyingstructure has a longitudinal demagnetization factor parallel to thetemporally constant magnetic field and a tangential demagnetizationfactor in a tangential direction transverse to the temporally constantmagnetic field. The longitudinal demagnetization factor is larger thanthe tangential demagnetization factor to produce tangential fluxguiding.

One advantage resides in reducing space consumption in a magneticresonance imaging scanner.

Another advantage resides in improved radio frequency coil operatingefficiency.

Yet another advantage resides in providing preferential flux guiding inthe tangential direction.

Still yet another advantage resides in reduced eddy current losses.

Numerous additional advantages and benefits will become apparent tothose of ordinary skill in the art upon reading the following detaileddescription of the preferred embodiments.

The invention may take form in various components and arrangements ofcomponents, and in various process operations and arrangements ofprocess operations. The drawings are only for the purpose ofillustrating preferred embodiments and are not to be construed aslimiting the invention.

FIG. 1 diagrammatically shows a magnetic resonance imaging systemincluding a magnetic field-modifying structure disposed between theradio frequency shield and rungs of a birdcage radio frequency coil.

FIGS. 2A and 2B diagrammatically show end and side sectional views of aradio frequency birdcage coil with a magnetic field-modifying structuredisposed between the birdcage rungs and the radio frequency screen, inwhich the magnetic field-modifying structure includes ferromagneticparticles dispersed in an insulating binder.

FIGS. 3A and 3B diagrammatically show end and side sectional views of aradio frequency birdcage coil with a magnetic field-modifying structuredisposed between the birdcage rungs and the radio frequency screen, inwhich the field-modifying structure includes elongated ferromagneticwires or rods dispersed in an insulating binder.

FIGS. 4A and 4B diagrammatically show end and side sectional views of aradio frequency birdcage coil with a magnetic field-modifying structuredisposed between the birdcage rungs and the radio frequency screen, inwhich the field-modifying structure includes generally planarferromagnetic plates dispersed in an insulating binder.

FIGS. 5A and 5B diagrammatically show end and side sectional views of aradio frequency birdcage coil with a magnetic field-modifying structuredisposed between rungs of the birdcage coil, in which the magneticfield-modifying structure includes ferromagnetic particles dispersed inan insulating binder.

FIGS. 6A and 6B diagrammatically show end and side sectional views of aradio frequency birdcage coil with a magnetic field-modifying structuredisposed between rungs of the birdcage coil, in which the magneticfield-modifying structure includes ferromagnetic annular rings shaped topromote tangential flux guiding.

FIG. 6C shows a perspective view of one of the ferromagnetic annularrings of FIGS. 6A and 6B, along with indications of the demagnetizationfactors in the z-direction and in the tangential direction.

With reference to FIG. 1, a magnetic resonance imaging scanner 10includes a housing 12 defining a generally cylindrical scanner bore 14inside of which an associated imaging subject 16 is disposed. Mainmagnetic field coils 20 are disposed inside the housing 12, and producea temporally constant main B_(o) magnetic field directed generally alongand parallel to a central axis 22 of the scanner bore 14. The centralaxis 22 lies parallel to the z-direction in the reference x-y-zCartesian coordinate system indicated in FIG. 1; however, othercoordinate systems can be used. For example, a vertical magnet can beused, in which the temporally constant B₀ field is vertically orientedin the y-direction. The main magnetic field coils 20 are typicallysuperconducting coils disposed inside cryoshrouding 24, althoughresistive permanent magnetic main magnets can also be used.

The housing 12 also houses or supports magnetic fieldgradient-generating structures, such as magnetic field gradient coils30, for selectively producing magnetic field gradients parallel to thecentral axis 22 of the bore 14, along in-plane directions transverse tothe central axis 22, or along other selected directions. The housing 12further houses or supports a radio frequency body coil 32 forselectively exciting magnetic resonances. Specifically, the radiofrequency body coil 32 produces a radio frequency B₁ magnetic fieldtransverse to the static main B₀ magnetic field. The radio frequency B₁magnetic field is generated at the Larmor frequency for exciting nuclearresonances. For exciting ¹H proton nuclei, the magnetic resonance Larmorfrequency f_(res) generally corresponds to f_(res)=γB₀ where γ=42.58MHz/Tesla is the gyrometric ratio for the ¹H nuclei and B₀ is the staticmain B₀ magnetic field. Thus, for example, at B₀=3 T, f_(res)=128 MHz.While ¹H proton nuclei exist in high concentrations in the human bodyand are commonly used for magnetic resonance imaging, other nuclearmagnetic resonances can be similarly excited and imaged.

In the illustrated embodiment, the coil 32 is a birdcage coil. A coilarray 34 is optionally disposed inside the bore 14 to receive magneticresonance signals. The coil array 34 includes a plurality of coils,specifically four coils in the illustrated example coil array 34,although other numbers of coils can be used, including a single surfacecoil. Moreover, the optional coil array 34 can be omitted altogether,and the body coil 32 used for receiving magnetic resonance signals. Thehousing 12 typically includes a cosmetic inner liner 36 inside thebirdcage coil 32 defining the scanner bore 14.

The main magnetic field coils 20 produce the main B₀ magnetic fieldparallel to the z-direction in the bore 14. A magnetic resonance imagingcontroller 40 operates magnetic field gradient controllers 42 toselectively energize the magnetic field gradient coils 30, and operatesa radio frequency transmitter 44 coupled to the radio frequency coil 32to selectively energize the radio frequency coil 32. By selectivelyoperating the magnetic field gradient coils 30 and the radio frequencycoil 32, magnetic resonance is generated and spatially encoded in atleast a portion of a region of interest of the imaging subject 16. Byapplying selected magnetic field gradients via the gradient coils 30, aselected k-space trajectory is traversed during acquisition of magneticresonance signals, such as a Cartesian trajectory, a plurality of radialtrajectories, or a spiral trajectory. Alternatively, imaging data can beacquired as projections along selected magnetic field gradientdirections. During imaging data acquisition, the magnetic resonanceimaging controller 40 operates a radio frequency receiver 46 coupled tothe coils array 34, as shown, or coupled to the whole body coil 32, toacquire magnetic resonance samples that are stored in a magneticresonance data memory 50.

The imaging data are reconstructed by a reconstruction processor 52 intoan image representation. In the case of k-space sampling data, a Fouriertransform-based reconstruction algorithm can be employed. Otherreconstruction algorithms, such as a filtered backprojection-basedreconstruction, can also be used depending upon the format of theacquired magnetic resonance imaging data. The reconstructed imagegenerated by the reconstruction processor 52 is stored in an imagememory 54, and can be displayed on a user interface 56, stored innon-volatile memory, transmitted over a local intranet or the Internet,viewed, stored, manipulated, or so forth. The user interface 56 can alsoenable a radiologist, technician, or other operator of the magneticresonance imaging scanner 10 to communicate with the magnetic resonanceimaging controller 40 to select, modify, and execute magnetic resonanceimaging sequences.

To stretch the static main B₀ magnetic field, to improve uniformity ofthe main B_(o) magnetic field, or to otherwise modify or configure themain B₀ magnetic field, a magnetic field-modifying structure 60 whichincludes a plurality of ferromagnetic annular rings, specifically eightannular rings 62 in the embodiment illustrated in FIG. 1, are disposedin and designed to enhance the static main B₀ magnetic field. Whileeight annular rings 62 are illustrated, other numbers and/or placementsof rings can be used. Moreover, a magnetic field-modifying structureincluding partial rings, rods, or other ferromagnetic structures may beemployed. Typically, the number, distribution, shape, and othergeometrical characteristics of the rings 62 of the magneticfield-modifying structure 60 are selected during concurrent design ofthe magnet 20. For example, these characteristics of the magneticfield-modifying structure 60 are suitably optimized during a finiteelement modeling optimization of the static magnetic field produced bythe magnet 20 in conjunction with the magnetic field-modifying structure60.

In FIG. 1, the magnetic field-modifying structure 60 is disposed betweenthe radio frequency coil 32 and a radio frequency shield 64 of the radiofrequency coil 32. In this position, it overlaps and interacts with theradio frequency B₁ magnetic field generated by the radio frequency coil32. At typical magnetic resonance frequencies, such as the example 128MHz resonance frequency for ¹H protons in a 3T static magnetic field,penetration of the radio frequency B₁ magnetic field into ferromagneticmaterials is limited to a skin depth of typically about 10-20 microns orless. For example, using f_(res)=128 MHz, relative magnetic permeabilityμ_(r)˜1 since the ferromagnetic material is in saturation, and aconductivity σ˜1×10⁷ S/m, the skin depth δ(f_(res)) is approximately:$\begin{matrix}{{{{\delta( f_{res} )} \cong \frac{1}{\sqrt{\pi\quad f_{res}\mu_{r}\mu_{o}\sigma}}} = {14\quad{microns}}},} & (1)\end{matrix}$where μ_(o)=4π=1⁻⁷ H/m is the magnetic permeability of free space andthe product μ_(r)μ_(o) is the absolute magnetic permeability of theferromagnetic material. The magnetic field in a ferromagnetic particlehaving dimensions substantially larger than the skin depth issubstantially expelled from the interior of the ferromagnetic particle.Such magnetic flux expulsion can adversely affect the performance of theradio frequency coil 32.

With reference to FIGS. 2A and 2B, in a first embodiment the magneticfield-modifying structure 60 ₁ (where the subscript “1” on certainreference numbers of FIGS. 2A and 2B indicate components specific to thefirst described embodiment of the magnetic field-modifying structure 60) has ferromagnetic annular rings 62 ₁ which are made up offerromagnetic particles 70 ₁ dispersed in an insulating binder 72. Inone example embodiment, the ferromagnetic particles 70 ₁ are pure ironparticles, iron alloy particles such as iron-cobalt alloy particles, orthe like, and the binder 72 is an electrically insulating non-magneticmaterial such as a polymer, resin or the like.

To substantially reduce flux expulsion of the radio frequency B₁magnetic field from the ferromagnetic particles 70 ₁, the particles aregenerally smaller in at least one dimension (for example, at least oneof length, width, and depth, or an annular cross-sectional dimension ofring-shaped particles) than a skin depth of the radio frequency B₁magnetic field in the ferromagnetic material, to allow the radiofrequency B₁ magnetic field to enter the ferromagnetic particles 70 ₁.The phrase “generally smaller than” the skin depth recognizes that theparticles 70, may have a statistical size distribution in which someparticles may be larger than the skin depth. In such cases, thestatistical distribution is such that most of the particles are smallerin the at least one dimension than the skin depth, so that fluxexpulsion is substantially reduced.

The ferromagnetic annular rings 62 ₁ include ferromagnetic particles 70₁ that generally do not have a direction of elongation, and are thusgenerally smaller than the skin depth of the radio frequency B₁ magneticfield in the ferromagnetic material in all dimensions. The fluxexpulsion decreases as the size of the ferromagnetic particles 70 ₁decreases. In one specific embodiment, the ferromagnetic particles 70 ₁are generally smaller than about one-tenth of the skin depth of theradio frequency field. In another specific embodiment, the ferromagneticparticles 70 ₁ are generally smaller than about 10 microns, whichcorresponds to the skin depth of typical ferromagnetic materials attypical magnetic resonance frequencies. In yet another specificembodiment, the ferromagnetic particles 70 ₁ are generally smaller thanabout 4 microns, which corresponds to about one-third of the skin depthof typical ferromagnetic materials at typical magnetic resonancefrequencies.

The fill factor of the ferromagnetic particles 70 ₁ dispersed in thebinder 72 should be high enough to provide the desired magnetic fieldmodification of the static B₀ magnetic field. In one embodiment, thefill factor is at least about 50% by volume. The fill factor of aspecific embodiment determines the ferromagnetic properties of theannular rings 62 ₁. The fill factor is, in turn, used in designing themagnetic field-modifying structure. The magnetic field-modifyingstructure 60 ₁ is designed to enhance the main B₀ magnetic field. Thisdesign can be performed concurrently with design of the main magneticfield coils 20, for instance by a finite element model optimizationincorporating both the magnetic field coils 20 and the magneticfield-modifying structure 60 ₁. Alternatively or in addition, thestructure 60 ₁ or portions thereof can be designed empirically, forexample by empirical shimming of the manufactured magnet to correct thestatic B₀ magnetic field for manufacturing flaws. Regardless of how andwhen the design is performed, the design of the magnetic field modifyingstructure 60 ₁ incorporates the specific ferromagnetic properties of theferromagnetic particles 70 ₁ dispersed in the binder 72.

With reference to FIGS. 3A and 3B, in a second embodiment, the magneticfield-modifying structure 602 (where the subscript “2” on certainreference numbers of FIGS. 3A and 3B indicate components specific to thesecond described embodiment of the magnetic field-modifying structure60) has ferromagnetic annular rings 62 ₂ which are made up of elongatedferromagnetic particles 70 ₂, such as rods, cigar-shaped particles, orwires of ferromagnetic material, dispersed in an insulating binder 72.The elongated ferromagnetic particles 70 ₂ can be, for example, ironfilings or iron whiskers. The magnetic field-modifying structure 60 ₂ ofFIGS. 3A and 3B differs from the magnetic field-modifying structure 60 ₁of FIGS. 2A and 2B in that the geometrically isotropic ferromagneticparticles 70 ₁ of FIGS. 2A and 2B have been replaced by elongatedferromagnetic particles 70 ₂ shown in FIGS. 3A and 3B.

To substantially reduce flux expulsion of the radio frequency B₁magnetic field from the ferromagnetic particles 70 ₂, a cross-sectionaldimension (for example, the wire diameter in the case of round wires) ofthe elongated ferromagnetic particles 70 ₂ are generally smaller thanthe skin depth of the radio frequency B₁ magnetic field in theferromagnetic material. In one specific embodiment, the elongatedferromagnetic particles 70 ₂ have cross-sectional dimensions that aregenerally smaller than about one-tenth of the skin depth of the radiofrequency field. In another specific embodiment, the cross-sections aregenerally smaller than about 10 microns. In yet another specificembodiment, the cross-sections of the particles 70 ₂ are generallysmaller than about 4 microns. The fill factor of the ferromagneticparticles 70 ₂ dispersed in the binder 72 is at least about 50% byvolume.

In FIGS. 3A and 3B, the elongated ferromagnetic particles 70 ₂ are shownsubstantially aligned with the tangential direction (designated by acurved arrow labeled “T” in the drawings). The tangential direction isspatially dependent. The tangential direction is everywhere transverseto the z-direction. The tangential direction is also at each point inspace transverse to a radial direction parallel to the x-y plane anddirected from the central axis 22 to that point in space.

The tangential alignment of the elongated ferromagnetic particles 70 ₂can be achieved, for example, by dispersing the elongated ferromagneticparticles 70 ₂ in the binder with the binder in a liquid form, andapplying an aligning magnetic field while the binder is cured orotherwise solidified. As will be discussed later, the tangentialorientation of the elongated ferromagnetic particles 70 ₂ shown in FIGS.3A and 3B can provide advantageous magnetic flux guiding. However, inother contemplated embodiments, the orientation of the elongatedferromagnetic particles 70 ₂ is substantially random.

Moreover, FIG. 3A illustrates that the annular rings 62 can bediscontinuous. For example, FIG. 3A shows a gap 66 in the rings 62 ₂.Although gaps such as the gap 66 can be included, for flux-guidingembodiments such gaps should be relatively few, and each gap should benarrow.

With reference to FIGS. 4A and 4B, in a third embodiment, the magneticfield-modifying structure 60 ₃ (where the subscript “3” on certainreference numbers of FIGS. 4A and 4B indicate components specific to thethird described embodiment of the magnetic field-modifying structure 60) has ferromagnetic annular rings 62 ₃ which are made up of generallyplanar ferromagnetic particles 70 ₃, such as plates or disks offerromagnetic material, dispersed in an insulating binder 72. It will beappreciated that the magnetic field-modifying structure 60 ₃ of FIGS. 4Aand 4B differs from the magnetic field-modifying structure 60 ₁ of FIGS.2A and 2B in that the geometrically isotropic particles 70 ₁ of FIGS. 2Aand 2B have been replaced by the generally planar particles 70 ₃ shownin FIGS. 4A and 4B.

To substantially reduce flux expulsion of the radio frequency B₁magnetic field from the generally planar ferromagnetic particles 70 ₃,the thickness of the generally planar ferromagnetic particles 70 ₃ aregenerally smaller than the skin depth of the radio frequency B, magneticfield in the ferromagnetic material. In one specific embodiment, thegenerally planar ferromagnetic particles 70 ₃ have thicknesses that aregenerally less than about one-tenth of the skin depth of the radiofrequency field. In another specific embodiment, the thicknesses aregenerally less than about 10 microns. In yet another specificembodiment, the thicknesses of the particles 70 ₃ are generally lessthan about 4 microns. The fill factor of the ferromagnetic particles 70₂ dispersed in the binder 72 is at least about 50% by volume.

In FIGS. 4A and 4B, the generally planar ferromagnetic particles 70 ₃are shown with planar normals substantially aligned parallel with thez-direction which corresponds to the direction of the main B₀ magneticfield, and transverse to the tangential direction. Such alignment can beachieved, for example, by dispersing the generally planar ferromagneticparticles 70 ₃ in the binder with the binder in a liquid form, andapplying an aligning magnetic field while the binder is cured orotherwise solidified. As will be discussed later, the orientation of thegenerally planar ferromagnetic particles 70 ₃ shown in FIGS. 4A and 4Bcan provide advantageous magnetic flux guiding. However, in othercontemplated embodiments, the orientation of the generally planarferromagnetic particles 70 ₃ is substantially random. Random orientationof the generally planar ferromagnetic particles 70 ₃ is particularlysuitable when the cross-sectional area of the generally planarferromagnetic particles 70 ₃ is relatively small.

With reference to FIGS. 5A and 5B, in a fourth embodiment, the magneticfield-modifying structure 60 ₄ (where the subscript “4” on certainreference numbers of FIGS. 5A and 5B indicate components specific to thefourth described embodiment of the magnetic field-modifying structure 60) has ferromagnetic annular rings 62 ₄ which are made up of generallygeometrically isotropic planar ferromagnetic particles 70 ₄, such asparticles similar to the particles 70 ₁ of the first embodiment of FIGS.2A and 2B, dispersed in an insulating binder 72. The ferromagneticannular rings 62 ₄ differ from the annular rings 62 ₁ of the firstembodiment in that the annular rings 62 ₄ are disposed at about the sameradial position (respective to the central axis 22) as the rungs of thebirdcage coil 32. To accommodate the overlap between the annular rings62 ₄ and the rungs of the birdcage coil 32, the ferromagnetic annularrings 62 ₄ each include gaps 68 in which the rungs are disposed. Inother words, the ferromagnetic material of the annular rings 62 ₄ isdisposed between the rungs of the birdcage coil 32. This arrangementenables the gap between the rungs of the birdcage coil 32 and the radiofrequency screen 64 to be narrowed versus the first embodiment.Alternatively, continuous annular ring portions can extend around therungs of the birdcage coil radially inward of the rungs, outward of therungs, or both.

With reference to FIGS. 6A, 6B, and 6C, in a fifth embodiment, themagnetic field-modifying structure 60 ₅ (where the subscript “5” oncertain reference numbers of FIGS. 6A, 6B, and 6 C indicate componentsspecific to the fifth described embodiment of the magneticfield-modifying structure 60 ) has ferromagnetic annular rings 62 ₅which are made up of ferromagnetic material not dispersed as particlesin a binder. For example, the ferromagnetic annular rings 62 ₅ may besolid or laminated iron rings, iron-alloy rings, or rings of anotherferromagnetic material. The rings 62 ₅ preferably form complete circuitsin the tangential direction; however, one or a few narrow gaps such asthe gap 66 may be included.

The magnetic field-modifying structure 60 ₅ includes annular rings 62 ₅that promote tangential flux guiding. The magnetic field H_(obj) in anferromagnetic object responsive to an applied external field H_(ext) isgiven by:H _(obj) =H _(ext) −NM _(sat)   (2),where M_(sat) is the saturation magnetization and N is thedemagnetization factor. The term NM_(sat) is called the demagnetizationfield, and for ferromagnetic materials is directed opposite to theapplied external field H_(ext). The saturation magnetization M_(sat) isa characteristic of the material, and the demagnetization factor N is acharacteristic of the physical geometry of the object.

For example, a spherical object has an isotropic demagnetization factorN which is independent of direction. A wire- or rod-shaped object has ademagnetization factor component of about zero for applied externalfield directed parallel to the wire or rod, and a non-zerodemagnetization factor component for applied external field directedparallel to the wire or rod. A generally planar object has ademagnetization factor component of about zero for in-plane directions,and a non-zero demagnetization factor component in the direction of theplanar normal, that is, transverse to the plane. In general, thedemagnetization factor N has larger components in directions of smallspatial extent, and smaller components in directions of large spatialextent.

With particular reference to FIG. 6C, the ferromagnetic annular rings 62₅ have a thickness d_(z) in the z-direction which is thin relative tothe width d_(r) of the annular ring 62 ₅ in a radial directiontransverse to the tangential direction. The extent of the annular ring62 ₅ in the tangential direction for a continuous ring or for a ringwith one or a few narrow gaps 66 is larger than either the thicknessd_(z) or the width d_(r). Because of the small thickness d_(z) relativeto the extended nature of the annular rings 62 ₅ in the tangentialdirection, the demagnetization factor component N_(z) in the z-directionis substantially larger than the demagnetization factor component N_(T)in the tangential direction. That is, N_(z)>>N_(T). This is indicated inFIG. 6C by using a thin short arrow to designate NT and a long thickarrow to indicate N_(z). For an isotropic saturation magnetizationM_(sat), therefore, the magnetic flux in the annular rings 62 ₅ ispreferentially guided in the tangential direction. In the z-direction,the relatively large N_(z) subtracts from the flux producing reducedmagnetic flux in the z-direction.

To provide a numerical example, for N_(T)=0 due to the large tangentialextent, a relatively large demagnetization factor component N_(z)=0.5 inthe z-direction due to the small thickness d_(z), and a ferromagneticmaterial having a saturation magnetization M_(sat)=2 T/μ_(o), applyingEquation (2) in the z-direction gives:H _(obj,z) =H _(ext,z) −N _(z) M _(sat) =H _(ext,z)−1 T/μ _(o)   (3).Applying Equation (2) in the tangential direction gives:H _(obj,T) =H _(ext,T) −N _(T) M _(sat) =H _(ext,T)   (4).Equations (3) and (4) show that the z-component of the magnetic field inthe annular rings 62 ₅ is suppressed by the subtractive factor 1T/μ_(o), whereas the tangential component of the magnetic field is notsuppressed, producing preferential flux guiding in the tangentialdirection. If the result of Equation (3) drops below 1 T/μ_(o), then thematerial is not in saturation in the z-direction.

The annular rings 62 ₅ of the flux-guiding fifth embodiment are made upof ferromagnetic material not dispersed as particles in a binder. Inother contemplated flux-guiding embodiments, the ferromagnetic rings maybe made of ferromagnetic particles dispersed in a binder. For example,the annular rings 62 ₁, 62 ₂, 62 ₃ of the first, second, and thirdembodiments, respectively, can provide flux guiding if the annular ringsare made thin in the z-direction compared with the width of the rings inthe radial direction. In contemplated embodiments, the rings are lessthan a few centimeters thick in the z-direction, and more preferably area few millimeters thick in the z-direction, to provide a substantialdemagnetization factor component in the z-direction.

The second embodiment 60 ₂ has elongated ferromagnetic particles 70 ₂that are advantageously oriented to promote the tangential flux guiding.The elongated direction of the elongated ferromagnetic particles 70 ₂ isparallel to the tangential direction, which results in a smalltangential demagnetization factor component. In the z-direction, thetangentially oriented elongated ferromagnetic particles 70 ₂ present athin dimension which enhances the demagnetization factor component inthe z-direction, thus suppressing the z-component of the magnetic fieldin the particles 70 ₂. Thus, if the annular rings 62 ₂ are designed tobe thin in the z-direction relative to a width of the rings in theradial direction, the annular rings 62 ₂ typically provide tangentialflux guiding.

Similarly, the third embodiment 60 ₃ has generally planar ferromagneticparticles 70 ₃ that are advantageously oriented to promote thetangential flux guiding. The tangential direction lies in the plane ofthe generally planar ferromagnetic particles 70 ₃, which results in asmall tangential demagnetization factor component. The planar normal ofthe generally planar ferromagnetic particles 70 ₃ lies along thez-direction, so that the particles 70 ₃ are thin in the z-directionwhich enhances the demagnetization factor component in the z-direction,thus suppressing the z-component of the magnetic field in the particles70 ₃. Thus, if the annular rings 62 ₃ are designed to be thin in thez-direction relative to a width of the rings in the radial direction,the annular rings 62 ₃ typically provide tangential flux guiding.

The annular rings 62 ₄ of the fourth embodiment generally providelimited tangential flux guiding since the rings are broken by the gaps68. If the segments of the annular rings 62 ₄ between the gaps 68 areextended in the tangential and radial directions compared with thethickness of the annular rings 62 ₄ in the z-direction, some tangentialflux guiding may be achieved.

Embodiments of the magnetic field-modifying structure 60 which promotetangential flux guiding of the radio frequency B₁ field will alsoproduce some preferential tangential flux guiding of the magnetic fieldgradients produced by the magnetic field gradient coils 30. Since themagnetic field gradients are imposed on the main B₀ magnetic field whichis directed in the z-direction, the magnetic field gradients typicallyhave small or non-existent components in the tangential direction at theposition of the magnetic field-modifying structure 60. Moreover,tangential flux guiding of the gradient fields can be further suppressedby including the magnetic field-modifying structure 60 in the design ofthe gradient coils 30. For example, the magnetic field-modifyingstructure 60 can be incorporated into a finite element modeloptimization of the gradient coils geometry.

Although the example magnetic field-modifying structures 60 have beendescribed with reference to a horizontal closed cylindrical magnet 20,the described embodiments are readily adapted to other magneticresonance imaging scanners such as vertical magnet scanners, asymmetricscanners, open scanner geometries, and the like.

The invention has been described with reference to the preferredembodiments. Obviously, modifications and alterations will occur toothers upon reading and understanding the preceding detaileddescription. It is intended that the invention be construed as includingall such modifications and alterations insofar as they come within thescope of the appended claims or the equivalents thereof.

1. A magnetic resonance imaging scanner comprising: a magnet generatinga temporally constant magnetic field; one or more magnetic fieldgradient-generating structures superimposing selected magnetic fieldgradients on the temporally constant magnetic field; a radio frequencyshield; a radio frequency coil disposed inside of the radio frequencyshield and selectively producing a radio frequency field; and a magneticfield-modifying structure designed to enhance the temporally constantmagnetic field, the magnetic field-modifying structure being disposedinside of the radio frequency shield and including particles of magneticmaterial generally smaller in at least one dimension than a skin depthof the radio frequency field in the magnetic material dispersed in aninsulating binder.
 2. The magnetic resonance imaging scanner as setforth in claim 1, wherein the particles of magnetic material dispersedin the hinder have a fill factor of at least about 50% by volume.
 3. Themagnetic resonance imaging scanner as set forth in claim 1, wherein theparticles of magnetic material are generally smaller in at least onedimension than about one-tenth of the skin depth of the radio frequencyfield in the magnetic material.
 4. The magnetic resonance imagingscanner as set forth in claim 1, wherein the particles of magneticmaterial are generally smaller than about 10 microns in at least onedimension.
 5. The magnetic resonance imaging scanner as set forth inclaim 1, wherein the particles of magnetic material are generallysmaller than about 4 microns in at least one dimension.
 6. The magneticresonance imaging scanner as set forth in claim 1, wherein the particlesof magnetic material generally do not have a direction of elongation. 7.The magnetic resonance imaging scanner as set forth in claim 1, whereinthe particles of magnetic material are generally wire-shaped.
 8. Themagnetic resonance imaging scanner as set forth in claim 7, wherein thegenerally wire-shaped particles flare oriented with long directionsgenerally transverse to the temporally constant magnetic field andgenerally parallel to a tangential direction.
 9. The magnetic resonanceimaging scanner as set forth in claim 1, wherein the particles ofmagnetic material are generally planar.
 10. The magnetic resonanceimaging scanner as set forth in claim 9, wherein the generally planarparticles are oriented with plane normals generally parallel to thetemporally constant magnetic field.
 11. The magnetic resonance imagingscanner as set forth in claim 1, wherein the radio frequency coilincludes a plurality of parallel rungs, and the particles of magneticmaterial are disposed at least partially between the rungs.
 12. Themagnetic resonance imaging scanner as set forth in claim 1, wherein themagnetic field-modifying structure includes: a plurality of generallyannular structures containing particles of magnetic material, thegenerally annular structures being oriented genera transverse to thetemporally constant magnetic field, the annular structures havingannular cross-sections elongated transverse to the temporally constantmagnetic field.
 13. The magnetic resonance imaging scanner as set forthin claim 1, wherein the magnetic field-modifying structure includes: aplurality of magnetic generally annular structures containing theparticles of magnetic material in the insulating binder, the magneticgenerally annular structures being oriented generally transverse to thetemporally constant magnetic field, the magnetic annular structureshaving a longitudinal demagnetization factor parallel to the temporallyconstant magnetic field and a tangential demagnetization factor in atangential direction transverse to the temporally constant magneticfield, the longitudinal demagnetization factor being larger than thetangential demagnetization factor to produce tangential flux guiding.14. The magnetic resonance imaging scanner as set forth in claim 1,wherein the magnetic field-modifying structure has a longitudinaldemagnetization factor parallel to the temporally constant magneticfield and a tangential demagnetization factor in a tangential directiontransverse to the temporally constant magnetic field, the longitudinaldemagnetization factor being larger than the tangential demagnetizationfactor to produce tangential flux guiding.
 15. A magnetic resonanceimaging, scanner comprising: a magnet generating a temporally constantmagnetic field; one or more magnetic field gradient-generatingstructures superimposing selected magnetic field gradients on thetemporally constant magnetic field; a radio frequency coil selectivelyproducing a radio frequency field; and a magnetic field-modifyingstructure designed to enhance the temporally constant magnetic field,the magnetic field-modifying structure having a longitudinaldemagnetization factor parallel to the temporally constant magneticfield and a tangential demagnetization factor in a tangential directiontransverse to the temporally constant magnetic field, the longitudinaldemagnetization factor being larger than the tangential demagnetizationfactor to produce tangential flux guiding.
 16. The magnetic resonanceimaging scanner as set forth in claim 15, wherein the magneticfield-modifying structure includes: a plurality of generally annularstructures oriented generally transverse to the temporally constantmagnetic field, the annular structures having annular cross-sectionselongated transverse to the temporally constant magnetic field.
 17. Themagnetic resonance imaging scanner as set forth in claim 15, wherein themagnetic field-modifying structure includes: ferromagnetic particlesthat are generally smaller than a skin depth of the radio frequencyfield in the magnetic material in at least one dimension; and aninsulating binder in which the ferromagnetic particles are dispersed.18. The magnetic resonance imaging scanner as set forth in claim 17,wherein the ferromagnetic particles are dispersed in the binder with afill factor greater than about 50% by volume.
 19. The magnetic resonanceimaging scanner as set forth in claim 17, wherein the ferromagneticparticles have an anisotropic particle demagnetization factor with alargest particle demagnetization factor component generally oriented inthe direction of the temporally constant magnetic field and a smallerparticle demagnetization factor component oriented in a tangentialdirection transverse to the direction of the temporally constantmagnetic field.